1. Field of the Invention
The present invention relates generally to a method and an apparatus for reading out a synchronizing signal from a recording medium and, more particularly, is directed to a method and an apparatus for reading out a synchronizing signal provided in a data portion of each sector stored in a recording medium such as an optical disc, a read only type optical disc or a write-in-once (WIO) type optical disc, a magneto-optical disc.
2. Description of the Prior Art
Recently, various reading methods and reading apparatuses have been proposed for reading out data recorded on an optical disc provided as a data recording medium.
In accordance with these reading methods, by reading out an address from a storage area (ID portion) in which an address of a sector is stored, it is determined whether or not the sector is a sector (target sector) containing data to be read out. If it is the target sector, then data recorded in a data storage area (data portion) following the ID portion is read out next. Data is read out from the data portion on the basis of the sync. signal and re-sync. signal recorded in it.
More specifically, the format of each sector on the optical disc is composed of an ID portion 10 and a data portion 11, as shown in FIG. 1. The data portion 11 comprises of a sync. data portion (SYNC) 11a at its head, a number of divided data portions 11b composed of 15 bytes or 20 bytes are formed and a re-sync. data portion (RESYNC) 11c is formed between the divided data portions 11b. Data recorded in the data portion 11 is recorded in a distributed manner in the respective divided data portions 11b. Sync. pattern data which becomes a sync. signal is recorded on the sync. pattern data portion 11a and re-sync. pattern data which becomes a re-sync. signal is recorded on the re-sync. data portion 11c. Then, on reading the data of the sector, data in the starting divided data portion 11b is read out on the basis of the data (sync. signal) in the sync. data portion 11a and data in the succeeding divided data portion 11b is read out on the basis of data (re-sync. signal) in the re-sync. data portion 11c. That is, data in the respective divided data portions 11b are read out at the detection timings of the sync. signal and the re-sync. signal.
Accordingly, data in the data portion 11 cannot be read out without detecting the sync. signal and the re-sync. signal.
However, conventionally, since the duration of the detection periods of the sync. signal and the re-sync. signal are determined beforehand, the sync. signal or the re-sync. signal must be detected within its detection period. Then, detection starting and ending timings of the limited detection period of the succeeding re-sync. signal are determined by the detection of the sync. signal. Thus, if the sync. signal cannot be detected, the succeeding re-sync. signal cannot be detected. There is then the substantial disadvantage that data cannot be read out from the optical disc at all.
As the recording density of the optical disc is 1 bit/.mu.m.sup.2 which is more than one digit bigger comparing with magnetic disc or magnetic tape, it is apt to be affected by micro fault and many bit errors are apt to occur. The bit error rate is about 10.sup.-5 (one reading error per 10.sup.5 bits) which is 4 to 5 digits larger comparing with recording media such as magnetic disc. Therefore, in particularly, in an optical disc in which a read error occurs more frequently than with other recording media, frequent occurrence of such read errors of the sync. signal becomes a serious problem.
As described above, as the burst error is likely to occur in the optical disc comparing with the other recording media, data recording in the optical disc is performed such that the data (composed of user data and a control word) is divided into bits or word units and then is recorded by changing their orders. This is called interleave and by adapting this method burst errors are changed into random errors or short word errors, thereby the correcting capacity being increased. In case of data recording by interleave method, firstly an error check code called CRC (Cyclic Redundancy Check) or CRCC (Cyclic Redundancy Check Code) is formed in each one sector from data row of each interleave, then an error check code (ECC) composed of Read Solomon product code or Read Solomon Long Distance Code are generated based on above data and above CRC in each interleave. By recording by the interleave adding ECC in addition with the above CRC correcting capacity of the data is increased and thus bit error rate is decreased until 10.sup.-12.
By forming CRC and ECC at each interleave and by recording the CRC and the ECC together with the data on the optical disc by interleave, when 1 sector is composed of 10 24 bytes it is possible to restore the correct data even if 80 byte errors occur in one sector of the data. Further, when 1 sector is composed of 512 bytes it is possible to restore to the correct data even if 40 bytes errors occur in one sector of the data. Therefore, it is important to detect the succeeding re-sync. signal even if detection of a sync. signal is failed for increasing the reliability of the data of the optical disc.